In Denmark, introduction of iodine-fortified salt was associated with an initial rise, then a subsequent decline, in cases of overactive thyroid, or hyperthyroidism, researchers report.
The thyroid is a butterfly-shaped gland in the neck that produces hormones that regulate the body's metabolism. In hyperthyroidism, the gland is overactive, causing symptoms such as excessive sweating, heat intolerance, and nervousness.
Before implementing a nationwide program to distribute iodine-fortified salt, Denmark was an area of mild to moderate iodine deficiency, Dr. Charlotte Cerqueira, from Glostrup University Hospital, and colleagues note in the Journal of Clinical Endocrinology and Metabolism.
While the benefit of iodine fortification is beyond question in areas of severe iodine deficiency, they note, the benefit of iodine fortification in areas of mild to moderate iodine deficiency has to be weighed against the risks, which stresses the importance of monitoring its effects on the population.
To gauge the impact of the iodine-fortification program in Denmark on thyroid activity in the population, Cerqueira's team tallied dispensing of antithyroid medication before and after Denmark started an iodine-fortification program.
Of nearly 5.3 million Danes, 4,281 started using antithyroid medication in 1997. The researchers tallied antithyroid medication use again after the program began voluntarily in 1998, and became mandatory in 2000.
In the region of the country with moderate iodine deficiency, the number of antithyroid medication users increased 46 percent during the 4 years after fortification. This was most common among residents younger than 40 years and older than 75 years.
By contrast, in the mildly-deficient region of Denmark, antithyroid use increased by just 18 percent during the first 4 years, and only among residents 59 years of age and younger.
After 6 years of iodine fortification, the rates of antithyroid use began to decline to the level they were prior to the start of fortification.
Transient iodine-induced hyperthyroidism has been reported in the early phases of almost all iodine fortification programs, Cerqueira and colleagues point out in their report.
SOURCE: Journal of Clinical Endocrinology and Metabolism, July 2009